The present invention relates to a control system for a continuously Variable transmission (CVT) for a motor vehicle, and more particularly, relates to a fail-safe system for the CVT in case of a speed change signal defect.
In recent years, control systems for a continuously variable transmission, including control valves for secondary and primary pressures, are electronically operated. Further, transmission ratio control is intended to be optimized by using the secondary pressure according to transmission torque and by using the primary pressure according to various driving and traveling conditions. An electronic control system of the above type has been proposed, the system having a secondary control valve in the form of a relief valve having a proportional electromagnetic solenoid and a primary control valve in the form of a pressure reducing valve having a proportional electromagnetic solenoid. The secondary control valve releases a part of the pump discharge pressure in accordance with a solenoid current and proportionally controls the secondary pressure to a predetermined value. On the other hand, the primary control valve reduces the primary pressure in accordance with a solenoid current and proportionally controls the primary pressure to a desired value.
Since the above-mentioned continuously variable transmission electronically controls the primary and secondary pressures by using electric signals, the transmission is provided with a fail-safe function against such defects as breakage of cables and solenoids. The primary control valve for changing the transmission ratio is a proportional electromagnetic pressure reducing valve of pressure feedback type. Thus, when no electric current flows, the primary pressure becomes maximum or minimum depending on the characteristics of the primary control valve so that the transmission will be forcedly shifted up or down, respectively. Consequently, as the fail-safe operation, such speed change situations should also be considered so as to enable the transmission ratio to be safely conducted.
Japanese Patent Application Laid-Open No. 91-189466 discloses an electronic control system for the above-mentioned continuously variable transmission, with respect to fail-safe countermeasures against no-electric current state of the primary control valve for changing the transmission ratio. The proportional electromagnetic pressure reducing valve is of a pilot-controlled pressure feedback type. While the vehicle is traveling, if the electric current is stopped and the primary pressure decreases to a minimum level, then clamp force will be reduced so that belt slippage will occur or the transmission will be abruptly shifted down, causing a wheel lock. To prevent such occurrences, the characteristics of the valve are determined such that if no-current state takes place, the primary pressure will become maximum and thereby the transmission will be shifted to the highest position.
Nevertheless, in the above-mentioned prior art, the transmission ratio is controlled in such a way that when the electric current is stopped the primary pressure will be forcibly increased to a maximum level and the transmission ratio will be fixed to a minimum level. In the prior art, although such occurrences as belt slippage and wheel lock are prevented, the start-up characteristics are remarkably deteriorated since the vehicle must be started with the minimum transmission ratio of the transmission. In addition, in such a start-up condition, the clutch may be damaged or excessively heated. Thus, the durability of the clutch is deteriorated.